Mitigating leaks in membranes

ABSTRACT

Two-dimensional material based filters, their method of manufacture, and their use are disclosed. In one embodiment, a membrane may include an active layer including a plurality of defects and a deposited material associated with the plurality of defects may reduce flow therethrough. Additionally, a majority of the active layer may be free from the material. In another embodiment, a membrane may include a porous substrate and an atomic layer deposited material disposed on a surface of the porous substrate. The atomic layer deposited material may be less hydrophilic than the porous substrate and an atomically thin active layer may be disposed on the atomic layer deposited material.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.14/530,292, filed Oct. 31, 2014, which claims the benefit under 35U.S.C. § 119(e) of U.S. provisional application Ser. No. 61/898,779,filed Nov. 1, 2013, the disclosures of which are incorporated byreference in their entirety.

GOVERNMENT LICENSE RIGHTS

This invention was made with government support under Grant No.DE-SC0008059 awarded by the Department of Energy. The government hascertain rights in the invention.

FIELD

Disclosed embodiments are related to mitigating leaks in membranes

BACKGROUND

Many industries and applications, such as water purification, chemicalsynthesis, pharmaceutical purification, refining, natural gasseparation, and many other applications rely on energy-intensivemembrane separation as a major component of their processes. The needfor membranes with high selectivity and flux for both liquid-phase andgas-phase membranes has led to many improvements in ceramic andpolymer-based membranes over the past few decades. One of the primarychallenges has been maximizing flux while maintaining high selectivity.Typically, increasing flux rate necessitates a decrease in selectivity.While several decades of research has resulted in development ofpolymeric or ceramic membranes, further advances in membrane technologywill likely rely on new membrane materials that provide better transportproperties. Recent advances in two-dimensional (2D) materials such asgraphene have opened new opportunities to advance membrane technology,where these 2D materials can form the active layer that confersselectivity.

SUMMARY

In one embodiment, a membrane may include an active layer including afirst plurality of defects and a deposited material associated with thefirst plurality of defects. The material may reduce flow through thefirst plurality of defects, and a majority of the active layer may befree from the material.

In another embodiment, a method of forming a membrane may includedepositing a material to reduce flow through a first plurality ofdefects in an active layer where a majority of the layer is free fromthe material.

In yet another embodiment, a method of forming a membrane may include:depositing a material using atomic layer deposition onto a poroussubstrate where the material is less hydrophilic than the poroussubstrate; and bonding an atomically thin active layer to the poroussubstrate.

In another embodiment, a membrane may include a porous substrate and anatomic layer deposited material disposed on a surface of the poroussubstrate. The atomic layer deposited material may be less hydrophilicthan the porous substrate. The membrane may also include an atomicallythin active layer disposed on the atomic layer deposited material.

It should be appreciated that the foregoing concepts, and additionalconcepts discussed below, may be arranged in any suitable combination,as the present disclosure is not limited in this respect. Additionally,the foregoing and other aspects, embodiments, and features of thepresent teachings can be more fully understood from the followingdescription in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

The accompanying drawings are not intended to be drawn to scale. In thedrawings, each identical or nearly identical component that isillustrated in various figures may be represented by a like numeral. Forpurposes of clarity, not every component may be labeled in everydrawing. In the drawings:

FIG. 1 is a schematic flow diagram of a method for forming a membrane;

FIG. 2 is a schematic perspective view of sealing a defect in an activelayer;

FIG. 3A is a schematic cross-sectional view of sealing a defect in anactive layer using an interfacial reaction;

FIG. 3B is a schematic cross-sectional view of sealing a defect in anactive layer using an interfacial reaction;

FIGS. 4A-4B are a schematic representation of an interfacial reactionselectively sealing defects greater than a predetermined size;

FIG. 5 is a schematic cross-sectional view of sealed defects in anactive layer using atomic layer deposition;

FIG. 6 is a schematic cross-sectional view of an active layer includingmultiple layers;

FIG. 7 is a schematic perspective cross-sectional view of sealing adefect in an active layer disposed on a substrate;

FIG. 8 is a schematic cross-sectional view of a membrane including anactive layer disposed on a substrate;

FIG. 9A is a schematic cross-sectional view of the membrane of FIG. 8after sealing the defects using an interfacial reaction;

FIG. 9B is a schematic cross-sectional view of the membrane of FIG. 8after sealing the defects using atomic layer deposition;

FIG. 10 is a schematic cross-sectional view of a membrane including anactive layer disposed on a substrate coated with an atomic layerdeposited material;

FIG. 11 is a graph of selectivity versus etch time for membranes sealedwith interfacial polymerization and unsealed membranes;

FIG. 12 is a graph of percent increase in selectivity ratio versus etchtime for a membrane sealed with interfacial polymerization as comparedto unsealed membranes;

FIG. 13A is a graph of KCl leakage for a bare polycarbonate track-etched(PCTE) membrane substrate, after graphene has been disposed on thesubstrate, after ALD deposition of HfO₂, and after interfacialpolymerization;

FIG. 13B is a graph of helium flow rate for a bare polycarbonatetrack-etched membrane substrate, graphene disposed on a substrate,graphene disposed on a substrate and subjected to interfacialpolymerization, and a bare substrate subjected to interfacialpolymerization;

FIG. 14A is an SEM image of graphene disposed on top of an 1 am porepolycarbonate track etched membrane;

FIG. 14B is an SEM image of graphene disposed on top of an 1 am porepolycarbonate track etched membrane;

FIG. 15A is an SEM image of 5 nm thick atomic layer deposited aluminumoxide on graphene disposed on top of an 1 am pore polycarbonate tracketched membrane

FIG. 15B is an SEM image of 5 nm thick atomic layer deposited aluminumoxide on graphene disposed on top of an 1 am pore polycarbonate tracketched membrane;

FIG. 16A is an SEM image of 10 nm thick atomic layer deposited aluminumoxide on graphene disposed on top of an 1 am pore polycarbonate tracketched membrane;

FIG. 16B is an SEM image of 10 nm thick atomic layer deposited aluminumoxide on graphene disposed on top of an 1 am pore polycarbonate tracketched membrane;

FIG. 17A is an SEM image of 15 nm thick atomic layer deposited aluminumoxide on graphene disposed on top of an 1 am pore polycarbonate tracketched membrane;

FIG. 17B is an SEM image of 15 nm thick atomic layer deposited aluminumoxide on graphene disposed on top of an 1 am pore polycarbonate tracketched membrane;

FIG. 18 is a graph of normalized helium flow rate versus thickness ofatomic layer deposited aluminum oxide;

FIG. 19 is an SEM image of 15 nm thick atomic layer deposited aluminumoxide on graphene disposed on top of an 1 am pore polycarbonate tracketched membrane;

FIG. 20 is an SEM image of 15 nm thick atomic layer deposited aluminumoxide on graphene disposed on top of a 10 nm pore polycarbonate tracketched membrane;

FIG. 21 is a graph of normalized helium flow rate versus thickness ofatomic layer deposited aluminum oxide;

FIG. 22 is a graph of helium flow rate through a 10 nm porepolycarbonate track etched membrane before and after graphene transferfor unmodified substrates and substrates that have been modified withatomic layer deposited hafnium oxide;

FIG. 23A is a visible light microscopy image of graphene including adefect on a PCTE after an interfacial reaction;

FIG. 23B is fluorescence microscopy image of graphene including a defecton a PCTE after an interfacial reaction, where the interfacial polymeris fluorescently labeled;

FIG. 23C is a fluorescence microscopy image of a cross section takenalong line 23C of FIG. 23B;

FIGS. 23D-23E are graphs of mean pixel fluorescence values forinterfacial polymerization on bare PCTE, Graphene on PCTE, and Grapheneon PCTE treated with HfO₂ ALD where the interfacial polymer isfluorescently labeled;

FIG. 23F-23H are fluorescence microscopy images corresponding to FIGS.23D-23E.

FIG. 24 is a graph of the flux and rejection rates for a compositemembrane for various sized molecules and ions.

DETAILED DESCRIPTION

The inventors have recognized that two-dimensional atomically-thinmaterials including a single, or in some instances several, atomiclayers, have immense potential as a highly-permeable, highly-selectivefiltration membranes. Due to the ability to create angstrom andnanometer scale pores in a single sheet of these materials, twodimensional materials have the ability to effectively and efficientlypermit selective transport of molecules for filtration in liquid and gasseparation processes. Additionally, and without wishing to be bound bytheory, the ultrathin thicknesses associated with these materials maypermit extremely high permeance and corresponding flow rates whilemaintaining better selectivity as compared to less-organized polymericmembranes. However, in addition to the benefits associated with usingatomically thin membranes, the inventors have recognized that one of theprimary challenges facing their development for use is the presence ofintrinsic defects introduced during manufacturing of the material andextrinsic defects introduced during transferring processes of thesematerials onto porous support substrates, as well as larger poresintroduced during selective pore formation processes. These defectstypically exhibit a broad size distribution which can range from about 1nm up to several nanometers, or even from micrometers to millimeters,and may allow free transport of most ions, solutes, and gases across anactive layer made from these materials. Further, unlike membranes offinite thickness, atomically thin materials are more susceptible toleakage through these defects and will correspondingly exhibit decreasednet selectivity when these types of defects are present.

In view of the above, the inventors have recognized that it may bedesirable to reduce, or eliminate, leakage through the noted defects.Additionally, it may also be beneficial to provide a membrane activelayer including monodisperse pores in applications such asnanofiltration, reverse osmosis, and other membrane applications toprovide a desired level of selectivity. Consequently, the inventors haverecognized the benefits associated with selectively depositing amaterial into the pores of a supporting substrate and/or on or withinthe defects in an active layer to reduce flow through these nonselectiveleakage paths. As described in more detail below, in some ofembodiments, atomic layer deposition, chemical vapor deposition, and/oran interfacial reaction may be used to deposit a selected material toreduce flow through the defects. Depending on the embodiment, thedefects may be sealed during the manufacturing process and/or during arepair after the membrane has been in service. Selective pores of adesired size may also be introduced into the active layer either priorto, or after, sealing the defects to provide a desired selectivity.

For the sake of clarity, the embodiments and examples described beloware primarily directed to use of graphene. However, the methods andmembranes described herein are not so limited. For example, appropriatematerials may also include hexagonal boron nitride, molybdenum sulfide,vanadium pentoxide, silicon, doped-graphene, graphene oxide,hydrogenated graphene, fluorinated graphene, covalent organicframeworks, layered transition metal dichalcogenides (eg. MoS2, TiS2,NiO2, etc.), layered Group-IV and Group-III metal chalcogenides (eg.SnS, PbS, GeS, etc), silicene, germanene, and layered binary compoundsof Group IV elements and Group III-V elements (e.g. SiC, GeC, SiGe), andany other appropriate atomically thin material. Additionally, in someembodiments the methods described herein may be applied to theproduction of thicker non-atomically thin membrane materials such asgraphene containing multiple atomic layers, graphene oxide containingmultiple atomic layers, metal organic frameworks, thin-layer atomiclayer deposition of metal oxides (AlO₂, HfO₂, etc), zeolites, and otherappropriate materials as well.

Without wishing to be bound by theory, these methods of reducing flowthrough defects in atomically thin materials may help to enable theproduction of highly selective membranes comprising a single atomicallythin selective layer without the need to stack multiple atomically thinlayers together to reduce leakage pathways. However, it should beunderstood that these methods may also be used with membranes includingmultiple layers of atomically thin materials. Additionally, while theembodiments described herein are directed to atomically thin materials,the disclosed methods could also be applied to membranes with finitethickness as well.

In other embodiments, atomic layer deposition of a material onto aporous substrate might be used to control the hydrodynamic resistance ofthe pores in the porous substrate to restrict flow through the defectsin an associated active layer. Again, without wishing to be bound bytheory, the ability to tune a substrate's pore diameter via atomic layerdeposition may help to improve selectivity of the overall resultingmembrane.

In yet another embodiment, atomic layer deposition of material onto aporous substrate may be used to modify the surface characteristics ofthe substrate to improve the transfer of an active layer onto thesubstrate. For example, the substrate may have a hydrophilic surface andthe atomically are deposited material may be more hydrophobic than thesubstrate surface to reduce wicking of an etchant between the substrateand an active layer during the transfer process. This may lead toenhanced bonding between the substrate and active layer as well asreduced leakage through the membrane due to the active layer notappropriately adhering to the substrate.

As will become more evident in view of the description below, thedisclosed methods and membranes may help to enable the use of atomicallythin materials as molecular sieves for gas-phase or liquid-phaseseparation processes by reducing or eliminating the permeance ofnon-selective, uncontrolled holes through mechanisms such as pluggingthe defects or minimizing their impact on membrane transport properties.Additionally, the disclosed methods of manufacture, and the resultingmembranes, may be applied to any number of different applications. Forexample, some commercial applications of the described membranesinclude, but are not limited to: water purification to remove pathogens,organic molecules, and salts (desalination/softening); portable waterfilters; preconcentrators for liquid or gas samples for use in sensingapplications; gas separation in energy applications such as natural gasseparation (methane from carbon dioxide, hydrogen sulfide, and heavierhydrocarbons) and carbon sequestration; dialysis in biological research;medical implants for allowing only select molecules to go through (e.g.,for sensor applications); controlled drug release devices; and in fuelcells as proton-selective membranes to name a few.

Turning now to the figures, specific non-limiting embodiments aredescribed in more detail. It should be understood that various featuresof the separately described embodiments may be used together as thecurrent disclosure is not limited to the specific embodiments depictedin the figures and described below.

FIG. 1 depicts a generalized flow diagram for manufacturing a membrane.More detailed descriptions of the various steps are provided below inreference to the other figures. In step 2, an active layer for use in amembrane is formed. The layer may be an atomically thin layer ofmaterial and may be formed using any appropriate technique. Afterforming the active layer, pores may optionally be formed in the activeplayer prior to transferring the active layer to a substrate, see 4.Depending on the embodiment, the pores may be sub-nanopores, nanopores,or micropores. The active layer may be transferred and subsequentlybonded to an appropriate substrate at 8 using any appropriate transfertechnique. However, in some embodiments, prior to transferring the layerto a substrate at 8, the substrate may be treated with an atomic layerdeposited material to increase the flow resistance of pores in thesubstrate and/or to alter the surface of the substrate to enhancebonding with the active layer deposited there on. After bonding theactive layer to the substrate, material may be deposited to reduce flowthrough defects present in the active layer at 10 using any appropriatemethod as discussed in more detail below. Additionally, the material mayeither be deposited within the defects, on top of the defects on theactive layer, within an associated substrate, or in any otherappropriate location depending on the deposition method used. While thisstep has been shown to be performed after transferring the active layerto the substrate, it is possible that depositing material to reduce flowthrough the defects could be performed prior to the active layer beingtransferred to the substrate. Additionally, in some embodiments, poresmay be formed in the active layer at 12 subsequent to the material beingdeposited to reduce flow through the defects, though as noted above andindicated by step 4, the pores me also be formed at an earlier step.Additionally, the pores may optionally be functionalized to enhance adesired characteristic of the resulting membrane at 14.

The general concept of depositing a material to reduce flow through adefect is illustrated in FIG. 2. As illustrated in the figure, an activelayer 100 includes a defect 102. As noted above, the defect may be onthe order of several nanometers up to, and possibly greater than,several micrometers. In order to reduce flow of a desired materialthrough the defect 102, for example hydrogen, a material is depositedinto, or on top of, the defect 102 to form a plug 104. In someinstances, the plug 104 may completely fill or cover the defect 102 toreduce a flow of a gas or liquid there through. However, in someinstances, the plug 104 may only partially fill the defect 102. In suchan instance, the defect 102 may be substantially filled such that areduction in the open area of the defect is still sufficient to reduce aflow of a desired gas or liquid there through. As illustrated in thefigure, the material used to form the plug 104 is preferentiallydeposited at the site of the defect 102 leaving the majority of theactive layer surface free of the deposited material. As described inmore detail below, the material used to form the plug 104 may bedeposited in any number of ways including, but not limited to, aninterfacial reaction, atomic layer deposition, and chemical vapordeposition.

In embodiments using an interfacial reaction, a polymer, mineral, or anyother solid deposit capable of reducing the flow of a desired gas orliquid is formed using a self-limiting chemical or precipitationreaction at the interface between two separate phases containingreacting monomers or components. Without wishing to be bound by theory,wherever the two separate phases contact one another, they form orprecipitate the desired material. Therefore, by controlling the locationof an interface between these two phases relative to the active layer,it is possible to control the location at which the material is formedor precipitated. For example, the interface may be located either on asurface of the active layer or within the active layer such that thedeposited material is deposited on, or in, the defects themselves.Additionally, to facilitate manufacture and use of these membranes, thedeposited material used to seal the defects may be insoluble in thefirst phase, the second phase, and/or a phase that the membrane will besubjected to during use.

One such embodiment is illustrated in FIG. 3A which depicts an activelayer 100 including a plurality of defects 102. The active layer 100 isarranged such that a first phase 106 is located on one side of thegraphene layer and a second phase 108 is located on an opposing secondside of the active layer. As noted above, the first phase reacts withthe second phase to form a precipitant or other product at theirinterface. If the two phases are not appropriately controlled, theinterface between the phases may be located past a surface of the activelayer 100 and the material formed may not be deposited in the desiredlocations. Parameters that may be used to control the location of theinterface include, but are not limited to, a pressure on either side ofthe active layer, a surface tension of the phases with the active layerand/or support substrate, a functionalization of the active layer and/orsupport substrate, concentrations or pressures of components in thephases, choice of solvent if performed in liquid phase or choice ofbackground inert gas if performed in gas phase, and a radius of thesupport substrate to name a few. For example, and without wishing to bebound by theory, functionalizing one side of the active layer to behydrophobic and the other side to be hydrophilic may be pin theinterface at the plane of the active layer. By appropriately using theabove-noted control parameters, the interface between the two phases maybe located either in, or on a surface of the active layer 100. Thus, thereaction, and the deposited material, may be restricted to places whereholes, cracks, or other defects 102 in the active layer allow the twophases to come into contact. The material formed or precipitated atthese locations seals the defects 102 with plugs 104. Because thereaction is restricted to where the defects are located, the remainingportion of the active layer 100 may be substantially free from thedeposited material. Selective nanopores, or pores with other desiredsizes, can then be introduced into the active layer to create a highlyselective filtration membrane.

In embodiments similar to the one discussed above, the location andability to seal a membrane using certain types of interfacial reactionsmay depend on the relative concentrations of the reactants. For example,the interfacial reaction of reactants having homobifunctional end groups(e.g. one monomer with amine end groups and another with acyl chloride)occurs where the fluxes of the two monomers have the correctstoichiometry. In the instance of aqueous and organic phase monomers,the aqueous phase monomer is typically soluble in the organic phase, andthe polymer is deposited in the organic phase. Without wishing to bebound by theory, if the monomers are denoted by x-A-x and y-B-y where xand y are reactive groups, the monomer to formed would be -A-B-A-B-.However, if the number of B monomers is much greater, for example morethan twice, the number of A monomers at a particular location, the Amonomers will tend to react with the excess B monomers to yieldy-B-A-B-y molecules that are unable to form longer polymer chains.Therefore, the polymer will form only when the fluxes of the reactantsare approximately matched to form a stoichiometric mix of reactants.While a reaction for monomers include two reactive groups have beendescribed above, the use of a stoichiometric mix of reactants tofacilitate the desired interfacial reaction may be applied to monomershaving more than two reactive groups as well as other types of reactantsthough the relative flux ratios of the reactants may be somewhatdifferent for different reactants.

In view of the above, if the fluxes of reactants used in an interfacialreaction are mismatched, the resulting polymer, or other material, mayform outside the composite membrane or it may not form at all. Forexample, if graphene with a 5 nm defect is suspended on polycarbonatepore membrane with 200 nm diameter pores, and an aqueous monomersolution x-A-x is introduced on the graphene side, it will haveinsufficient flux compared to the monomer y-B-y introduced on thepolycarbonate track-etched membrane side to form a stoichiometric mix ofreactants within the composite membrane. As a result, both reactivegroups of the monomer will be consumed and the result will be primarilythe formation of y-B-A-B-y instead of a polymer inside of the compositemembrane. However, if the flux of reactants within the compositemembrane controlled by appropriately controlling the transportresistances of the support filter versus the defects in the atomicallythin active layer the product from the mixture of the various reactantsmay be deposited within the composite membrane. Therefore, in someembodiments, the transport resistance of a supporting filter may begreater than or equal to the transport resistance of defects locatedwithin an active layer to facilitate formation of a stoichiometric mixof reactants within a composite membrane. For example, in the above caseof an active layer having 5 nm defects, a polycarbonate track-etchedmembrane with smaller 10 nm pores will decrease the flux of monomer B sothat the interfacial polymerization will be located within the compositemembrane and will favor the formation of the desired polymer.Alternatively, in some embodiments, a similar result may be obtained byincreasing the concentration of A and/or decreasing the concentration ofB to provide the desired flux of reactants within the compositemembrane.

While several specific embodiments to control the flux of reactants aredescribed above, it should be understood that any appropriatecombination of transport resistances of the active layer and/or supportas well as the relative concentrations of reactants in the variousphases may be used to provide a stoichiometric flux of reactants withinthe composite membrane to produce the desired interfacial reaction.

It should be noted that the interfacial reactions may be performed usingany number of monomers having two or more reactive groups. For example,in some embodiments, an interfacial reaction of a polyamide may beperformed using monomers such as amines and acyl chlorides. Appropriatemonomers that may be used include, but are not limited to, trimesoylchloride, polyhedral oligomeric silsesquioxane amine, phenylenediamine,propane-1,2,3-triamine, and adipoyl chloride.

In some instances, it may be desirable to perform an interfacialreaction without providing a stoichiometric flux of reactants within acomposite membrane. Therefore, in some embodiments, reactions that donot require a stoichiometric mixture of reactants to form the desiredinterfacial reaction may be used. For example, a phase includingmonomers, soluble polymers, and/or soluble molecules may be located onone side of an active layer of a composite membrane, and an agent thatcauses polymerization or precipitation of the monomers, solublepolymers, and/or soluble molecules may be located on the other side ofthe composite membrane. Depending on the embodiment, the molecules mayprecipitate or polymerize due to pH, the presence of a solvent, thepresence of catalysts, the presence of polymer chain growth initiator,or any other appropriate type of agent. In one specific embodiment,Poly(lactic acid) (PLA) is soluble in acetonitrile but not in water.Therefore, introducing PLA in acetonitrile on one side and water on theother side of a composite membrane will cause PLA to precipitate insidethe composite membrane. In yet another example, the formation ofpolyaniline in the presence of an oxidant [O] is as follows: nC₆H₅NH₂+[O]→[C₆H₄NH]_(n)+H₂O. Consequently, an oxidant such as ammoniumpersulfate may be introduced on one side of a composite membrane and themonomer may be introduced on the other. In yet another example,polypyrrole may be formed in a composite membrane using the oxidation ofpyrrole using ferric chloride in methanol.

FIG. 3B depicts another embodiment of using an interfacial reaction toseal an active layer 100 including a plurality of defects 102. In thedepicted embodiment, the active layer 100 is sequentially exposed to afirst phase 106 and a second phase 108. For example, as depicted in thefigure, the active layer 100 may be dipped into the first phase 106 suchthat the first phase 106 is wicked into the defects. However, in someembodiments, the first phase 106 may simply adhere to the surface of theactive layer 100 at the locations corresponding to the plurality ofdefects 102. Regardless of how the first phase 106 is held on the activelayer 100, when a side of the active layer 100 is exposed to the secondphase 108, the two phases react to form plugs 104 at the plurality ofdefects. While a particular arrangement for serially exposing the activelayer to the separate phases has been depicted in the figures anddescribed above, it should be understood that other arrangements forserially exposing the active layer are also possible.

Depending on the particular embodiment, the two reactive phases might bein the same state of matter, or different states of matter as thecurrent disclosure is not so limited. For example, both the first phaseand the second phase might be liquid. In another embodiment, one of thephases might be in a liquid state and/or a liquid phase that contains areactant which reacts with a gas to produce the desired material. Insuch an embodiment, the liquid phase may be provided on one side of theactive layer using any appropriate method such that it forms a plug whenit comes in contact with the gaseous second phase at the open pores anddefects of the active layer. Since graphene, is known to be impermeableto most gases in its defect-free state, this method should be relativelyeasy to implement as long as the pressure difference across themembranes is adequately controlled. In yet another embodiment, both thefirst phase and the second phase are gaseous phases.

The concept of performing an interfacial reaction to plug a plurality ofdefects in an active layer can be implemented using any number ofdifferent types of reactions including, but not limited to,precipitation reactions and interfacial polymerization reactions.Additionally, these reactions might be performed using two immisciblephases which may be enable the formation of highly stable andreproducible interfaces. For example, an interfacial polymerizationreaction using two immiscible phases may be used to produce a highlystable polymer layer that is several nanometers thick to seal thedefects and reduce or eliminate reducing or eliminate species transportacross the defects where the material is deposited. Additionally,depending on the particular interfacial reaction, material may bedeposited to reduce the flow through an associated defect on the sizescale of about 1 nm to several micrometers or more. While there arebenefits to using two immiscible phases, embodiments in which aninterfacial reaction is produced using two miscible phases are alsocontemplated.

While interfacial reactions may be performed using immiscible fluids, itshould be understood that an interfacial reaction may also be performedusing miscible fluids, or even the same fluids. These fluids may beintroduced on either side of a composite membrane including an activelayer, and in some embodiments a support membrane, that hinders mixingof the two fluids so that the stoichiometric fluxes of the reactantsoccur within the composite membrane. For example, if the graphene, orother active layer, on a support membrane has few defects, introducingmonomers in miscible fluids on either side of the composite membranewill lead to polymerization provided the fluxes yield the correctstoichiometry within the composite membrane.

In one embodiment, it may be desirable for an interfacial reaction toseal defects and/or pores that have been formed in a substrate above apredetermined size. For example, processes for creation of selectivepores in graphene typically result in pores with a distribution ofsizes. If the size distribution is wide enough, or there are otherlarger defects present within a substrate, it may result in poorselectivity. In some embodiments, the diameter of the larger poresand/or defects may be effectively decreased, or in some instances may beblocked entirely, by using molecules that are able to react, or “lock”,across a defect in the ultrathin cross section of a graphene or otheratomically thin substrate.

In the embodiment depicted in FIGS. 4A-4B, first and second phases 106and 108 located on either side of a graphene substrate 100 correspond totwo separate molecules. The graphene sheet includes a first defect 102above a preselected size and a second defect, or formed selective pore,103 with a size that is less than the preselected size. In the depictedembodiment, the two molecules include one end that is too large to passthrough the defects and selective pores in the graphene substrate.However, the molecules also include smaller ends that bind or react witheach other and are able to pass through the larger defects 102 but notthrough the smaller pores 103 in the graphene substrate. Therefore, themolecules are able to react and lock across defects having sizes greaterthan a size of the two reacting ends, such as defect 102, and are unableto react across defects having sizes less than the size of the tworeacting ends, such as selective pore 103. The result is that theinterfacial reaction using these molecules blocks, or reduces theeffective diameter, of pores or defects greater than a predeterminedsize while leaving pore or defects smaller than that size alone. In somecases, the small ends of the molecules may have a size ranging fromabout 0.3 nm to 10 nm, 0.3 nm to 5 nm, 0.3 nm to 1 nm, or any otherappropriate size. Additionally, the large ends of the molecules may havea size such that it is larger than, and in some embodiments the samesize as, the small end of the molecule and ranges from about 0.3 nm to20 nm, 1 nm to 20 nm, 5 nm to 20 nm, 10 nm to 20 nm, 1 nm to 10 nm, orany other appropriate size.

The above embodiment using reactants capable of “locking” across anatomically thin substrate may include any number of appropriate materialsystems. For example, the molecules may include a branched PEG(polyethyleneglycol) molecules, one with an alkyl amine terminal and theother with an acyl chloride terminal, e.g.(H(OCH₂CH₂)x)₃-C—(CH₂CH₂)y-NH₂ and (H(OCH₂CH₂)x)₃-C—(CH₂CH₂)y-COCl. Inanother embodiment, a PEG with one end conjugated to a large moleculesuch as a dextran and the other end terminated with biotin, andintroducing neutravidin on the other side of the reaction interfacemight be used. In yet another embodiment, (C₂H₅)₃—(CH₂)n-NH₂ and(C₂H₅)₃—(CH₂)n-COCl, where n is >2 and can penetrate across the poresare used. It should be understood that while particular reactants arelisted above, the specific reactants as well as the sizes of the largerand smaller ends of the molecules may be selected for any appropriateuse and for any appropriate size of defects as the current disclosure isnot so limited.

Without wishing to be bound by theory, there are several advantages tousing interfacial polymerization as a method to seal graphene compositemembranes. Many polymerization reactions happen at room temperaturewithout requiring heat or light, making their implementation easy,although reactions that require heat or irradiation could also be used.Second, polymerization reactions can be implemented in a flow throughsystem and do not require any special equipment. Hence, they are ideallysuited for large area membranes.

Since an interfacial reaction depends on the formation of a stableinterface and the transport of reactants through the active layer, it isbelieved that that the process may be implemented to only selectivelyseal a first plurality of tears, holes, and other defects that arelarger than a desired size while leaving a second plurality of smallertears, holes, and other defects unsealed. Without wishing to be bound bytheory, this might be accomplished by choosing an interfacial reactionincluding reactants larger than a particular size such that thereactants can only pass through, and thus seal, pores and defects thatare larger than the reactants while leaving smaller pores and defectsunsealed. Thus, the size of holes and defects that are sealed by aninterfacial reaction may be controlled by choosing reactants of acertain size. For example, different polymers with different lengthpolymer chains might be used to seal defects with different minimumsizes. However non-polymer reactants of a desired size might also beused. Due to the ability to only seal defects greater than a particularsize, interfacial reactions may be used during manufacture and/or usageof a membrane to seal defects larger than a size corresponding to theparticular reactants being used.

In some embodiments, it may be desirable to modify the properties ofmaterials deposited using an interfacial reaction. For example, apolymer may be formed with a range of functional groups and properties.In one embodiment, sulfhydryl groups may be incorporated into a polymerto facilitate grafting of additional polymers terminated with maleimidegroups in order to densify the polymer. In another embodiment, chargedgroups such as carboxyl acid may be included in a polymer to confercharge for modulating ionic transport. Other types of functionalizationmay also be used as the disclosure is not limited in this manner.

Depending on the embodiment, the interfacial reaction may only sealdefects that are larger than about 0.5 nm, 1 nm, 2 nm, 3 nm, 5 nm, or 10nm. For example, the interfacial reaction may be used to seal a membranewhile leaving sub-nanometer filtration holes intact. This may bebeneficial since most nanofiltration and reverse osmosis processesrequire holes that are in the sub-nanometer to several nanometer sizescale and the above-noted interfacial reactions may be used to seal thenonselective defects while leaving the desired selective pores unsealed.

Several examples of interfacial reactions include, but are not limitedto:

(a) an aqueous diamine (e.g. m-Phenylenediamine [MPDA] orHexamethylenediamine [HMDA]) and (b) an organic solution of an acylchloride (e.g. trimesoyl chloride [TMC] or adipoyl chloride [APC])yielding a polyamide at the interface;

(a) aqueous sodium sulfate and (b) aqueous calcium chloride yieldingcalcium sulfate at the interface;

(a) aqueous sodium carbonate and (b) aqueous calcium chloride yieldingcalcium carbonate at the interface;

(a) aqueous sodium carbonate and (b) aqueous zinc chloride yielding zinccarbonate at the interface;

(a) aqueous hydrochloric acid with ammonium persulfate and (b) anilinein chloroform yielding polyaniline at the interface;

(a) pyrrole in toluene and (b) aqueous ammonium persulfate yieldingpolypyrrole at the interface; and

(a) poly(lactic acid) in acetone, acetonitrile, or dichloromethane and(b) water form poly(lactic acid) precipitate.

FIG. 5 depicts another method for sealing a plurality of defects 102 inan active layer 100. In this particular embodiment, atomic layerdeposition (ALD) is used to deposit material onto the active layer 100to form the plugs 104. Typically ALD is a process in which alternatingcycles of adsorption of two different precursors is used to deposit athin oxide, or other material, one atomically thin layer at a time.Additionally, the inventors have recognized that ALD deposited materialpreferentially forms at defects in an active layer. Without wishing tobe bound by theory, the reason ALD material is preferentially formed atthe defect sites is because the ALD precursors are more easily adsorbedon defect sites as compared to pristine graphene that lacks polargroups. Without wishing to be bound by theory, ALD precursors arebelieved to be more easily adsorbed at defect sites because unsaturatedcarbon bonds are known to preferentially adsorb O₂, H₂O, and hydrocarboncontamination due to the defects having higher reactivity (surfaceenergy) than the rest of the lattice. Therefore, similar to interfacialreactions, ALD can be used to seal defects such as holes and tears in anactive layer while leaving significant areas of the active layer freefrom the deposited material. Again leaving the majority of the activelayer free from the deposited material allows the creation of selectivepores in the atomically thin material of the active layer. While ALD isdescribed above, other processes similar to ALD may be used. Forexample, any chemical vapor deposition process capable of depositing acontrolled thickness of material and selectively depositing thatmaterial at the defects as compared to the entire surface of an activelayer could be used to selectively seal the defects in the active layer.

ALD deposited layers may have any desired thickness. Additionally, dueto ALD depositing atomically thin layers of material during a singledeposition cycle, the thickness of the deposited material can be easilycontrolled by controlling the growth process. Appropriate parameters forcontrolling the growth of the ALD deposited layers include, but are notlimited to, temperature, pressure, duration of exposure, and number ofcycles. It should be understood that any appropriate combination ofparameters might be used. However in one embodiment, the number of ALDcycles may be between or equal to 5 cycles 30 cycles, 10 cycles and 20cycles, or any other appropriate number cycles. Further, the depositedmaterial may have a thickness between about 5 nm and 30 nm, 5 nm and 20nm, or 10 nm and 20 nm, or any other appropriate thickness.

Several nonlimiting examples of appropriate ALD reactions include, butare not limited to:

Alumina using Trimethylaluminum (TMA) and water;

Ruthenium oxide using Ru(od)3 and oxygen;

Zinc oxide using diethylzinc (DEZ) and water;

Titania using Ti[OCH(CH₃)₂]₄ and water;

Zirconia using Zr[OCH(CH₃)₂]₄ and water;

Titania using TiCl₄ and water;

Zirconia using ZrCl₄ and water; and

Hafnia using HfCl₄ and water.

The embodiments described above were directed to sealing membrane activelayers that included a single layer. However, the current disclosure isnot limited to sealing defects in a single layer. Instead, the disclosedmethods are capable of use on a membrane active layer including anynumber of layers. For example, FIG. 6 depicts an active layer 110 thatincludes two individual active layers 100. These individual activelayers 100 include a plurality of defects. In some instances, thedefects are aligned with one another as depicted by defects 102 a, andin other cases, the defects are unaligned with one another as depictedbe defects 102 b. The aligned defects 102 a permit material to passthrough a membrane without selectivity. In contrast, the unaligneddefects 102 b are blocked from permitting material to pass through themembrane by the adjacent pristine active layer 100. One embodiment inwhich multiple individual active layers 100 might be included to form anoverall active layer 110 is when the active layer is applied to asupporting substrate. More specifically, providing a plurality of activelayers may advantageously increase the covered area of the substratebecause when a plurality of active layers of the same size and shape areplaced on a substrate each will be randomly misaligned. However, it ishighly improbable that any would be misaligned in exactly the same way.Therefore, some of the area of the substrate left uncovered by oneactive layer would likely be covered by subsequently placed activelayers. Consequently, the uncovered area of the substrate may be reducedwhen a plurality of active layers are used. Other applications ofmultiple active layers are also possible. Additionally, while theindividual active layers 100 have been depicted as being in directcontact, in some embodiments, intermediate layers may be positionedbetween these adjacent active layers. Appropriate intermediate layersinclude: chemical cross-linkers with reactive terminal groups such asdiazonium; different polymers such as poly(dimethylsiloxane),polycarbonate, and polyamide; layers of atomic layer deposited materialsuch as alumina and hafnia; and other appropriate materials.

FIGS. 7-10 depict embodiments of an active layer 100 including aplurality of defects 102 disposed on a porous substrate 112. The poroussubstrate includes a plurality of pores 114. As depicted in the figures,the pores 114 are aligned pores similar to a track-etched membrane.However, porous substrates including unaligned random pore networks arealso possible. For example, graphene based filtration membranes, andother similar membranes, may be combined with a variety of supportingsubstrates including, but not limited to, porous ceramics, porousmetals, polymer weaves, nanofiltration membranes, reverse osmosismembranes, ultrafiltration membranes, brackish water filtrationmembranes, or any other appropriate substrate.

Depending on the particular embodiment, the porous substrate disposedbeneath the active layer may provide structural support to the membraneand may also impede flow through defects present in the one or moregraphene layers that are not occluded, or otherwise mitigated. Theporous support may provide sufficient resistance to flow through areaswhere large imperfections in the graphene exist, such that flow throughthe intended pores may still dominate the overall flow through thecomposite membrane. For example, the porous support may be apolycarbonate track-etched membrane with pore diameters in the range of5 nm to 10 μm, and pore lengths (i.e. support layer thickness) in therange of 1 μm to 5 mm (FIG. 3A). Alternatively, the porous support mightbe a ceramic support with pores in the size range of 10 nm to 10 μm, anda thickness in the range of 100 μm to 10 mm. Furthermore, the supportstructure itself may include multiple layers. For example, thepolycarbonate layer may rest on a sintered steel porous support.Furthermore, it should be understood that the graphene may be disposedon any other appropriate membrane or substrate. For example, asymmetricpolyamide membranes used for reverse osmosis of brackish water orseawater might be used. In such an embodiment, the pore sizes of themembrane may be less than 10 nanometers or less than 1 nanometer.

FIGS. 7, and 9A illustrate the application of an interfacial reaction toseal a plurality of defects 102 in an active layer 100 disposed on asubstrate 112. As illustrated in the figures, depending on where theinterface between the two reacting phases was located the defect 102 mayeither be sealed by plugs 104 b located in, or on, the defectsthemselves, or the defects 102 may be sealed by a plug 104 bcorresponding to material deposited in a pore 114 of the substrate 112that is associated with the defect. As noted above, the location of theinterface may be controlled in any number of ways. Therefore, it may bepossible to selectively form plugs in the active layer 100 itself and/orin the associated pores 114 of the porous substrate.

FIG. 9B illustrates the application of ALD to seal a plurality ofdefects 102 in an active layer 100 disposed on a substrate 112. Whenusing ALD to deposit material, material 116 will be deposited both atthe defects 102 in the active layer as well as the walls of the pores114 in the substrate. Different forms of defect sealing may occur usingALD depending on the size of the pores in the substrate as compared tothe size of the defects in the active player as well as the speed ofgrowth of the ALD deposited material in the defects and poroussubstrate. For example, the defects 100 may be sealed by a plug 104 bwhen the defects are sufficiently small or the speed of materialdeposition at the defects is sufficiently fast as compared to theposition of the material in the pores 114. Alternatively, if asupporting substrate's pores are sufficiently small and/or the rate ofdeposition of material on the substrate is sufficiently fast, thenmaterial 116 deposited in the pores 114 may either completely seal orrestrict flow through the defects 102. In both embodiments it isdesirable that that ALD growth in the defects and/or the pores of thesupporting substrate be sufficiently fast compared to lateral growth ofALD deposition on the active layer away from defect sites. Consequently,the majority of the active layer area may remain exposed followingtreatment using ALD.

One possible way in which to use ALD to seal the defects by depositingmaterial in the pores of the substrate includes protecting the backsideof the composite membrane from ALD and performing ALD on the exposedactive layer side. As illustrated in the figure, material 116 isdeposited in the exposed pores of the substrate associated with thedefects 102. However, because graphene is impermeable to ALD precursors,the ALD material will not be deposited in the covered pores 118 of thesubstrate. As noted above, depending on the ALD layer thickness, thedeposited material 116 can either completely seal or significantlyrestrict flow through the micrometer-scale tears in the compositegraphene membrane. Other methods of using ALD to seal pores in thesubstrate associated with defects in the active layer are also possible.

In some embodiments, ALD only covers defects in an active layer and doesnot substantially form on the defect free pristine areas of the activelayer. Without wishing to be bound by theory, ALD deposition iscontrolled by precursor adsorption. Additionally, precursor adsorptiondepends on several factors including, but not limited to, precursortype, pressure, temperature, and gases that compete for adsorption.Generally precursor adsorption decreases at higher temperatures.Therefore, adsorption performed at higher temperatures is more likely tospecifically target defects as compared to the pristine defect freeportions of the substrate. The introduction of gases such as nitrogenthat preferentially adsorb to graphene, or another type of active layer,can be used to make ALD deposition more specific to defects bypreventing the adsorption of ALD precursors to the defect free portionsof the active layer. Therefore, purging with nitrogen, or anotherappropriate gas such as methane, ethane, propane, benzene vapors,between precursors may increase the specificity of ALD coatings to thedefects by competitively desorbing weakly adsorbed precursors on thedefect free pristine portions of the active layer. In view of the above,the use of gases capable of adsorbing to the surface of an active layermay enable the use of ALD performed at lower temperatures while onlybeing deposited substantially at the defects of the active layer.

In one embodiment, ALD is performed on a graphene active layer using(Tetrakis)dimethylmino Hafnium and water. The process is performed at130 to 300° C. with a base pressure of 100-1000 mTorr using nitrogen asa purge gas. In one specific implementation of such an embodiment, 20cycles of HfOx are performed to precondition the chamber. 20 standardcubic centimeters (sccm) of nitrogen carrier gas are then applied for a2400 second purge time and a 0.025 second pulse of water is added to thechamber. After 50 seconds, 30 sccm of nitrogen are added to the chamber.After an additional 50 seconds, 20 sccm of nitrogen are added to thechamber over a period of 20 seconds. Thereafter, a 0.3 second pulse ofHf precursor is added to the chamber. After waiting 50 seconds, 30 sccmof nitrogen s are added to the chamber. After another 50 seconds, 20sccm of nitrogen are introduced to the chamber over 20 seconds. A final20 second rest is introduced between subsequent cycles. This cycle maybe applied for any appropriate number of times including between 10-20,20-30, 30-40, or any appropriate number of times as the disclosure isnot so limited. Additionally, while specific times, temperatures, purgetimes, and other parameters are described above, ALD processes may beperformed using other parameters.

In addition to sealing defects in an active layer, ALD can also be usedto tune the hydrodynamic resistance of a porous substrate. In such anembodiment, an ALD process may be carried out on the porous substrateprior to transferring the active layer in order to reduce the diameterof the pores. During this process, material is deposited onto thesurfaces of the pores thereby reducing their diameter and causing acorresponding increase in the flow resistance of the porous substrate.FIG. 10 shows a substrate 112 that includes pores 114 that have had amaterial 116 applied to them prior to bonding with an active layer 100.Without wishing to be bound by theory, the performance of a compositemembrane made from an active layer including nonselective defects may beimproved by appropriately matching the porous substrate resistance tothe active layer resistance. Due to the ability to precisely control thethickness of material deposited in the pores of the substrate, ALDoffers a method capable of precisely tuning the support membraneresistance to achieve a desired matching of the resistance of the poroussubstrate to the active layer. The feasibility of this approach isdemonstrated in more detail in the examples below.

It was observed that the typical hydrophilic coating applied tooff-the-shelf membranes to increase water flow rates complicatestransfers of centimeter-scale areas of chemical vapor deposited grapheneto the substrate. For example, polycarbonate track-etch membranes(PCTEM) are typically coated with hydrophilic polyvinylpyrrolidone(PVP). Without wishing to be bound by theory, these hydrophilic coatingscause a problem during transfer because during transfer the graphene islocated on a copper foil and is in contact with the support membranewhile the copper is chemically etched away. However, if the supportmembrane is too hydrophilic, such as when these hydrophilic coatingshave been applied, the etchant wicks in between the substrate and theactive layer, preventing the graphene from bonding to the support.Consequently, in some embodiments, the surface of a substrate may becoated with a coating that is sufficiently hydrophobic to ensure thatthe active layer bonds to the underlying substrate without the etchantwicking in between the substrate and the active layer. This coating maybe provided in any number of ways. However, in one embodiment, thecoating may be applied in a manner similar to that discussed above withregards to FIG. 10. In such an embodiment, ALD may be used to apply anappropriate material 116 to the surface, and optionally to the pores114, of a substrate 112. After depositing the material, an appropriatetransfer technique may be used to bond the active layer 100 to thesubstrate 112.

It should be understood that the above-noted methods and structures canbe used in combination to achieve a membrane with desiredcharacteristics. For example, ALD could be performed on a bare substrateto reduce the diameter of the pores in the substrate, followed bytransfer of the active layer onto the substrate. Another ALD step canthen be used to seal the various intrinsic and extrinsic defects presentin the active layer. Without wishing to be bound by theory, the smallersupport pore diameter resulting from the first ALD step might enable theuse of a smaller thickness ALD layer in the second step to seal pores inthe substrate that are associated with defects. Because the secondsealing step uses a smaller amount of ALD deposited material, the amountof lateral ALD growth over the active layer surface may be reduced thusleaving a larger fraction of the membrane area with exposed graphene inwhich selective pores can be produced. In another embodiment, ALD mightbe used on a polycarbonate track etched membrane to facilitatesubsequent graphene transfer, and then an interfacial reaction may beused to reduce non-selective leakage flow through the composite graphenemembrane. In yet other embodiments, defects of different sizes may besealed using a combination of an interfacial reaction and ALD. Forexample, ALD may be used to plug multiple small-scale defects and aninterfacial reaction could subsequently be applied to plug a smallernumber of remaining larger defects.

Depending on the embodiment, pores may be created in the active layereither prior to, or after, bonding the active layer to a substrate.Several options exist for precisely controlling the size of porescreated in the active layer. These include, but are not limited to, ionbombardment, chemical etching, gas cluster ion-beam bombardment, pulsedlaser deposition, plasma treatment, UV-ozone treatment, and growinggraphene on copper with patterned defects. Once the pores are generated,their sizes and shapes can be further refined through chemical etching.Additionally, intrinsic defects or pores in the synthesized graphene canbe used for filtration. These pores may occur naturally in chemicalvapor deposition (CVD) graphene, or may be introduced during synthesisof graphene by controlling the substrates on which the graphene isgrown. For example, the copper substrate for growing CVD graphene may bepatterned, alloyed, or coated with nanoparticles to facilitate theintroduction of defects of a desired size into the graphene duringgrowth. Additionally, gases such as ammonia or nitrogen may be addedduring synthesis to create pores during the CVD process. Furthermore,the amorphous regions in graphene may contain a higher number of pores,which can also be used for filtration. Regardless of the manner in whichthe defects are created, after forming the defects in the one or moreactive layers, the defects may be selectively etched to a preselectedsize. Examples of appropriate etchants for these materials include, butare not limited to, concentrated nitric acid, mixtures of potassiumpermanganate and sulfuric acid, hydrogen plasmas, and hydrogen peroxide.

As compared to the random distribution and alignment of the intrinsicpores, actively created pores may advantageously provide pores through,a single active layer, or multiple stacked active layers, in which thepores pass from one side of the active layer(s) to the other. Further,when these pores are created in a stack of active layers, the pores ineach active layer may be substantially aligned with one another.However, regardless of how the pores are generated, or whether the poresare present in a single active layer, or in a stack of active layers,the sizes and shapes of the pores may be controlled to create pore sizesappropriate for filtering molecules or particles of a particular size.This ability to provide pores that pass from one side of the activelayer(s) to the other in combination with the methods disclosed abovefor sealing undesired nonselective defects enables the production ofsubstantially defect free membranes with high selectivity. Additionally,since it is possible to seal these defects in a single atomically thinlayer, it may also be possible to realize the benefits associated withfilters made from these materials without the need to include a largenumber of multiple layers thus enabling larger fluxes while maintaininga desired selectivity.

In some embodiments, it may be desirable to modify a supporting membraneand/or active layer to enhance a selectivity of the deposition ofmaterials used to seal leaks from defects in the structure. In one suchembodiment, an active layer or substrate may be modified to facilitategrafting of polymers or polymerization on the surface. For example,plasma treatment may damage the surface of a polymeric membrane and makethe surface reactive, introduce free radicals, or introduce functionalgroups depending on the type of plasma used and the environment that themembrane is exposed to after plasma treatment. The plasma treatment canalso increase adhesion of an active layer to a substrate, which couldaid in graphene transfer from copper foil or another surface to thesubstrate. A surface may also be modified using irradiation by electronsor photons, e.g. UV modification. In some embodiments, the irradiationmay be performed selectively on a side of the membrane. Additionally, anactive layer and/or substrate may be modified using a variety offunctional groups. In one exemplary embodiment, a surface may bemodified to bind covalently or non-covalently to graphitic surfaces tofacilitate, for example, graphene transfer to a substrate. One possibletype of functionalization includes azide functionalization.

In one embodiment, a polysulfone, polyethersulfone ultrafiltrationmembrane, or other appropriate membrane used as a support substrate forgraphene is treated with carbon dioxide plasma followed by surfacefunctionalization with azide groups. Graphene is then transferred ontothe surface. Any tears, holes, or defects in the active layer will leavethe underlying support substrate with azide groups exposed. The exposedazide functional groups are then used for grafting or growth of polymers(e.g. by using an interfacial polymerization reaction) or usingnanomaterials (e.g. graphene nanoflakes, nanoparticles, dextrans) toblock leakage to the substrate that is exposed and left uncovered withgraphene.

In another embodiment, a polysulfone, polyethersulfone ultrafiltrationmembrane, or other appropriate membrane used as a support substrate forgraphene is treated with argon plasma, followed by transfer of graphenefrom copper foil. Any defects in the graphene will expose areas of thesupport membrane that are then modified with polymers, nanomaterials, orother materials. In embodiments where nanomaterials are used, thenanomaterial may be sized such that it is comparable to or larger insize than a pore size of the substrate. In other embodiments wherenanomaterials are used, the the nanomaterial may be sized such that itis smaller in size than a pore size of the substrate.

While the exemplary embodiments above have mostly detailedfunctionalizing the surface of the substrate, embodiments in which theactive layer edges and defects present include functional groups thatfacilitate grafting or growth of a material thereto are alsocontemplated.

In some embodiments, it may be desirable to increase the selectivity ofthe pores present in an active layer. Therefore, in such an embodiment,the pores present in the active layer may be functionalized to enhancethe selectivity of the composite membrane. For example, the pores mightbe functionalized such that they are hydrophobic or hydrophilicdepending on the desired application. Specific forms offunctionalization may include, but are not limited to, carboxyl groups,hydroxyl groups, amine groups, polymer chains (polyamide,polyethyleneglycol, polyamide, etc), small molecules, chelating agents,macrocycles, and biomolecules (e.g. crown ethers, porphyrins,calixarenes, deferasirox, pentetic acid, deferoxamine, DNA, enzymes,antibodies, etc.). In some embodiments, the above notedfunctionalizations, as well as other appropriate functionalizations, maybe used to modulate transport of a molecule or particle throughgraphene. For example, and without wishing to be bound by theory:15-crown-5 preferentially binds sodium ions and may thus regulate itstransport, or, it may regulate the transport of other ions or moleculesin response to binding of a sodium ion; polyethyleneglycol maypreferentially allow transport of only small hydrophilic molecules andions; and polyamide may allow for the preferential transport of water.In alternative embodiments, only the pores may be selectivelyfunctionalized. For example, the pores can have different chemicalgroups depending on the method of pore creation and treatment due to thepores oftentimes being more reactive than the surface of the activelayer. These differences can be used to selectively functionalize onlythe pores. Thus, embodiments in which the surface and/or pores of thegraphene are functionalized are possible.

In some embodiments, graphene, or another atomically thin material islocated on a substrate which may be modified using an appropriatechemistry such as a silane chemistry. For example, PEG-silanes where PEGis polyethylene glycol and the silane terminal is trichlorosilane,trimethoxysilane, etc. will graft onto the underlying substrate. In caseof graphene on gold-coated substrates, the substrates may be modifiedusing thiol chemistry, e.g. PEG-thiols. Several methods are availablefor modification of polyethersulfone and other polymer substrates bygrafting of other polymers, for example as outlined in Bhattacharya andMisra, Progress in Polymer Science 29 (2004) 767-814, or by Deng et alProgress in Polymer Science 34 (2009) 156-193, which are incorporatedhere in their entirety by reference. Other methods include blending ofpolymers to create specific functional groups for modification, e.g.covalent modification of polyethersulfone by bovine serum albumin byblending with poly(acrylonitrile-co-acrylic acid) as described by Fanget al Journal of Membrane Science 329 (March 2009), 46-55. Otherexamples are modification of poly(vinylidene fluoride) (PVDF) usingpoly(3,4-dihydroxy-1-phenylalanine) as described by Zhu et al Colloidsand Surfaces B: Biointerfaces 69, (2009) 152-155. Another example ismodification of cellulose substrates by surface initiated atom transferradical polymerization (ATRP) by growing poly[poly(ethylene glycol)methacrylate] layers as described by Singh et al Journal of MembraneScience 311 (2008) 225-234. Additionally, if reactants used in the aboveexamples are introduced only from the active layer side, surfacemodification of the substrate will occur only where defects in theactive layer permit reactants to cross across the active layer, therebycoating the underlying substrate and decreasing leakage.

In some embodiments, it is desirable to form a compact polymer layer.Therefore, in such an embodiment, especially for grafting lesswater-soluble polymers, the polymerization may take place in non-aqueousconditions in which the grafted polymer chains are flexible. The polymerwill subsequently collapse into a more compact form in the absence ofthe solvent after the polymerization has occurred.

For commercial applications, increasing the durability of the membranemay be desirable. Therefore, in some embodiments, a protective coatingmay be applied to the active layer to ensure that the membrane willfunction effectively after repeated handling and/or use. For example,the protective layer might be used to provide mechanical protectionand/or antifouling properties such as anti-scaling or anti-biofouling.Appropriate protective layers include, but are not limited to: polymersdeposited by layer-by-layer assembly such as polyethyleneglycol,polyamide, polysulfone, polyanionic and polycationic polymers;zwitterionic molecules; and nanoparticles such as silver and titaniananoparticles.

While many of the embodiments described above have been directed toselectively depositing materials only at the defects in an active layerand/or composite membrane, embodiments in which materials are depositingon the surface of the active layer and/or substrate are alsocontemplated. For example, a polymer may be covalently grafted, grown,or deposited onto an active layer as well as the underlying exposedsubstrate so long as it does not completely block flow through selectivepores formed in the active layer.

Example: Interfacial Polymerization Sample Preparation

Graphene was grown using a chemical vapor deposition (CVD) process andwas subsequently transferred to polycarbonate membranes with 200 nmdiameter pores using established protocols. The membranes were suspendedat the water/hexane interface in a Franz cell (Permgear). The aqueousphase contained diamine (m-Phenylenediamine [MPDA] orHexamethylenediamine [HMDA]) at a concentration of 10 mg/mL while theorganic phase contained an acyl chloride such as trimesoyl chloride[TMC] or adipoyl chloride [APC] at a concentration of 10 mg/mL. Thelower part of the cell was first filled with the aqueous solution, afterwhich the graphene composite membrane was slowly placed at theinterface, graphene side down, such that the aqueous phase wetted thegraphene side of the membrane. Then the top portion of the cell wascarefully placed and clamped in position. The organic phase (hexane) wasthen slowly added to the top cell (˜0.5 mL) to fill it half way and thereaction was allowed to proceed for the desired time (from 3 min up toseveral hours depending on the conditions). Afterward, the hexanesolution was carefully aspirated after which 0.5 mL of pure hexane wasadded to the top cell to wash away the remaining reactants. This washingprocess was repeated three times, after which the same washing procedurewas repeated with ethanol once (to neutralize the unreacted acylchloride). Lastly, the top cell was removed and the membrane was washedin a pure ethanol bath. The membrane was then placed in the test chamberor taken for further processing.

Example: Interfacial Reactions

In one set of experiments, graphene membranes were sealed using areaction between APC and HMDA, after which the resulting membranes wereprocessed to create tunable holes on the graphene. In this procedure,membranes comprising a layer of CVD graphene on 200 nm diameterpolycarbonate track etched membranes (PCTEM) were bombarded with focusedgallium ions to nucleate atomic defect sites and subsequently etched ina potassium permanganate and sulfuric acid solution to gradually grownanopores with etch time. The membranes were then loaded in diffusioncells and transport of hydrochloric acid and Allura Red was measuredusing established protocols. As illustrated by FIGS. 11-12, theinterfacial polymerization treatment improved the selectivity of themembranes as expected. In fact, a nearly two fold increase in theselectivity of the sealed membrane versus the unsealed membrane wasobserved at 10 minutes etch time.

In one set of experiments, the effect of interfacial polymerization (IP)using the diffusive transport rate of potassium chloride (KCl)normalized by transport rate through a bare PCTE membrane wereperformed. As shown by FIG. 13A, the transport rate decreases with eachsuccessive sealing step of adding graphene to the PCTE, applying HfO₂ tothe composite membrane using ALD and applying an interfacial reaction ofHMDA and APC. The final leakage rate of the composite membrane was 7%that of the untreated PCTE.

In a similar set of experiments, the amount of blockage the interfacialpolymerization treatment provided was determined by measuring thepermeance of the membranes to helium in a gas flow cell, see FIG. 13B.Note that the gas flow rates are normalized by the helium flow ratethrough a 200 nm pore polycarbonate track-etch membrane (PCTEM) withoutany graphene or interfacial polymerization. Normalized flow rates for amembrane including untreated graphene disposed on the PCTEM substrate,graphene disposed on the PCTEM substrate and treated with interfacialpolymerization, and a PCTEM substrate treated with interfacialpolymerization are also depicted. As illustrated by the graph, theinterfacial treatment on bare polycarbonate track etched membranes with200 nm pores resulted in a 99.96% reduction in flow rate, whileinterfacial treatment on graphene membranes (i.e. one layer of CVDgraphene transferred to the same type of polycarbonate membrane) causeda 96.78% reduction in flow as compared to the untreated graphenemembrane disposed on the same substrate.

In combination, these results demonstrate the ability of interfacialreactions to effectively reduce the flow of material through defects ingraphene membranes.

Example: Atomic Layer Deposition

Without wishing to be bound by theory, the spacing betweennanometer-scale intrinsic defects in CVD graphene is larger than thesmallest commercially available PCTEM support pore diameter (about 10nm). Additionally, the lateral growth rate of ALD on graphene away fromdefect sites has been measured to be several times greater than verticalgrowth rates. Consequently, scanning electron microscope (SEM) imageswere used in combination with gas flow measurements to demonstrate thatsupport membrane pore resistance can be significantly increased whileleaving large areas of exposed graphene. FIGS. 14A-17B show SEM imagesof aluminum oxide ALD of various thicknesses ranging from 0 nm to 15 nmdeposited on graphene disposed on 1 μm pore PCTEMs. These images wereobtained in a JEOL 6320FV Field-Emission High-Resolution SEM at anacceleration voltage of 5 kV and in secondary electron imaging mode.FIGS. 14A and 14B show a membrane before ALD, in which polycarbonatepores (1 μm diameter circles) are visible. Graphene sitting on top ofthe polycarbonate pores can be distinguished from the darker areas wherethe graphene is torn. FIGS. 15A-17B show membranes after ALD, withsuccessively higher numbers of ALD cycles. The concentration of thesmall white lines and spots, which are the deposited aluminum, increasewith the number of ALD cycles performed and show how the ALD grows onthe graphene composite membranes.

As illustrated in the figures, approximately 20% to 70% of the graphenearea remains uncovered by the ALD material when the layer thickness isbetween about 10 nm and 15 nm. The layers were deposited at 125° C.Additionally as shown by the graph in FIG. 18, ALD thicknesses betweenabout 10 nm and 15 nm lead to significant reductions in He gas flowthrough the membrane. In combination, these images and data show thatmicrometer-scale tears in a graphene membrane supported over 10 nmpolycarbonate pores can be almost completely sealed, while leaving largefractions of the graphene free to create selective nanometer-scalepores.

FIGS. 14A-17B also illustrate that the ALD material does not show apreference as to growing on graphene suspended over the pores ascompared to graphene located over the polycarbonate. This suggests thatALD will deposit similarly on membranes with smaller pore diameters.This is further supported by FIGS. 19 and 20 which show SEM images ofALD deposited aluminum oxide on graphene over a 1 μm pore PCTEM and overa 10 nm pore PCTEM. Although the 10 nm pores cannot be resolved, the ALDgrowth patterns as observed in the SEM images are similar for the twomembranes suggesting that the underlying PCTEM pores did not influencethe ALD deposition.

The existence of exposed graphene after ALD growth was further confirmedby exposing the membranes to oxygen plasma following ALD and measuringthe flow rate. FIG. 21 shows the normalized helium flow rates for a bare10 nm pore PCTEM as well as a membrane including a single graphene layerdisposed on PCTEM and a single graphene layer disposed on PC TM that wassubsequently exposed to a three-minute oxygen plasma etch. Normalizedflow rates greater than one in PCTEM's are common due to variability inmanufacturing. The flow rates for these different membranes areillustrated for a range of different ALD thicknesses. As expected, theflow rates through membranes with one layer of graphene that haveundergone ALD are decreased as compared to the flow rate through barePCTEMs that have undergone ALD, except in the case with 15 nm ALDthickness. Without wishing to be bound by theory, this is believed to bedue to an ALD thickness of 15 nm significantly blocking the PCTEM pores.However, a different behavior is observed for the membranes exposed tothe oxygen plasma etch. Without wishing the bound by theory, when agraphene layer is disposed on the substrate, some of the PCTEM pores areprotected during ALD deposition and material is not deposited there.Therefore, a greater He flow rate would be expected in the membraneafter the oxygen plasma etch as compared to the substrate simply exposedto ALD deposition because the previously protected, and consequentlyunblocked pores with lower flow resistance, are exposed when theoverlying graphene is etched away by the oxygen plasma etch. Thisbehavior is confirmed for ALD thicknesses of 10 nm and 15 nm where theHe flow rate is greater in the membrane including a graphene layer thathas been exposed to the oxygen plasma etch. This suggests that theoxygen plasma has removed the exposed graphene while leaving the ALDthat was within the substrate pores. Therefore, and again withoutwishing to be bound by theory, these results indicate that there was atleast a portion of the graphene that was not covered by the ALDmaterial.

Example: Controlling Permeance of the Porous Substrate

FIG. 21 demonstrates the viability of using ALD to control the permeanceof the porous substrate. More particularly, at ALD thicknesses betweenabout 10 nm and 15 nm, a significant reduction in the helium flow ratewas observed for the bare 10 nm pore PCTEM. As noted previously, the ALDprocess can be used to reduce the diameter of the pores in the poroussubstrate thereby reducing the flow through leakage pathways as well asprecisely and uniformly increasing the resistance to flow within themembrane. As illustrated by the bare PCTEM data, ALD can be used toincrease the resistance of the porous substrate by over one thousandtimes. Further, the observed gradual reduction in flow rate with ALDthickness confirms that the support membrane resistance can be tunedprecisely by controlling the ALD thickness.

Example: Using Atomic Layer Deposition to Modify Graphene Transfer

FIG. 22 is a graph of the helium flow rate through various membranes andillustrates the effectiveness of using ALD to modify the surface of asubstrate in order to achieve improved bonding with a graphene layerduring transfer. The tested membranes included: a bare 10 nm pore PCTEMsubstrate coated with polyvinylpyrrolidone (PVP); a 10 nm pore PCTEMsubstrate coated with PVP after a graphene transfer attempt; a bare 10nm pore PCTEM substrate subjected to 20 cycles of hafnium oxide ALD; anda 10 nm pore PCTEM substrate subjected to 20 cycles of hafnium oxide ALDafter a graphene transfer attempt. Without ALD, the graphene transferwas unsuccessful and no graphene was visible over the polycarbonate.This is confirmed by the large helium flow rate observed for thissample. However, after 20 cycles of hafnium oxide ALD, a transfer of a 5mm by 5 mm area of graphene to the substrate was successful asillustrated by the significantly lower helium flow rate through thatmembrane. The significantly lower flow rate after a graphene transferattempt on a PCTEM with ALD demonstrates that ALD can be used tofacilitate graphene transfers to PVP coated substrates. ALD is expectedto offer similar benefits for other coatings that graphene, or otheractive layers, do not bond readily bond to during typical manufacturingprocesses. In addition to the above, as illustrated by the comparisonbetween the bare PCTEM sample and the PCTEM sample after 20 cycles ofALD, the small 2 nm ALD thickness has almost no effect on the flow ratethrough the bare membrane.

Example: Labeled Interfacial Reaction

In one set of experiments, graphene membranes were sealed using nylon6,6 formed by the reaction between APC (5 mg/mL) and HMDA (5 mg/mL).Labeling the HMDA with a fluorescent dye (Texas Red-X SuccinimidylEster, Life Technologies) enabled the visualization of the position ofthe synthesized polymer through confocal fluorescence microscopy. Areflected light image and fluorescence images of the graphene membranereveal that fluorescently-labeled nylon 6,6 preferentially forms wheregraphene does not cover the PCTE pores, see FIGS. 23A-23C. Additionally,an analysis of the mean pixel florescence of the sample area revealedthat when graphene is present on the surface of the PCTE membrane, theamount of polymer coverage is significantly reduced to about 12% of thatmeasured for bare PCTE, see FIGS. 23D and 23E. This reduction indicatesthat graphene blocks the precipitation reaction unless defects permitit. However, when polymer does form behind the graphene, it does sonearer the graphene, suggesting the occurrence of intrinsic defectspermitting small amounts of the HMDA to leak across the graphene intothe organic side. As also shown by the figures, covering these intrinsicdefects with hafnium oxide via atomic layer deposition further reducespolymer formation as compared to the bare PCTE by about an additional5%. FIGS. 23F-23H are fluorescent images of a bare PCTE membrane, agraphene membrane, and a graphene membrane with HfO₂ deposition usingthe above noted fluorescent labeled nylon 6,6.

Example: Water Permeability

In another set of experiments, the permeability of water across themembrane described in the previous example under forward osmosis with adraw solution of 20 atm glycerol ethoxylate was measured. The membranewas formed by transfer of graphene, ALD of hafnia, interfacialpolymerization using HMDA and APC but without fluorescent label, andpore creation by ion bombardment and oxidative etching. To measure thepermeability, a graduated cylinder was affixed to the side of thediffusion cell containing deionized water and monitored over the courseof the experiment (20-40 min). Accounting for the exposed area of themembrane, the permeability of the nanoporous graphene was found to be0.88±0.19 L m⁻² h⁻¹ bar⁻¹. The experimental permeability agreed withtheoretical predictions within an order of magnitude.

Example: Salt and Organic Molecule Flux Rates

In yet another set of experiments, the flux of salts and organicmolecules using the membrane described in the previous example wasmeasured under forward osmosis with a draw solution of 20 atm glycerolethoxylate. Water permeability was calculated using the same procedureas described above. The concentration of the salts in the deionizedwater side were measured over time using a conductivity probe (eDAQIsoPod), while the concentration of the organic molecules were measuredover time using a UV-vis spectrophotometer (Cary 60). The measured ionsand molecules were NaCl (˜0.72 nm), MgSO₄ (˜0.8 nm), Allura Red AC (˜1.0nm), and 4.4 kDa Tetramethylrhodamine Dextran (˜2.3 nm).

As shown in FIG. 24, the composite membrane rejects the flow of largeorganic molecules, but allows for the transport of monovalent salts. Thenegative rejection, as defined as 1−{dot over (n)}_(sol)/{dot over(n)}_(sol) ^(o), where {dot over (n)}_(sol) is the experimentallymeasured solute flux and {dot over (n)}_(sol) ^(o) is the solute fluxwere the membrane non-selective, of the monovalent sodium chlorideindicates that leakage is still present. Qualitatively, the experimentalmass flux agrees with the theoretical predictions.

While the present teachings have been described in conjunction withvarious embodiments and examples, it is not intended that the presentteachings be limited to such embodiments or examples. On the contrary,the present teachings encompass various alternatives, modifications, andequivalents, as will be appreciated by those of skill in the art.Accordingly, the foregoing description and drawings are by way ofexample only.

What is claimed is:
 1. A method of forming a membrane, the methodcomprising: depositing a first material onto a surface of a poroussubstrate, wherein the first material is less hydrophilic than theporous substrate; and bonding an atomically thin active layer to thefirst material to bond the atomically thin active layer to the poroussubstrate, wherein the first material is disposed between the atomicallythin active layer and the porous substrate, wherein the atomically thinactive layer includes a first plurality of open pores, and wherein theporous substrate includes a second plurality of open pores that passthrough the porous substrate and the first material such that the firstplurality of pores and the second plurality of pores are in fluidcommunication such that a gas and/or liquid is capable of flowingthrough the membrane.
 2. The method of claim 1, wherein the atomicallythin active layer comprises at least one of graphene, hexagonal boronnitride, molybdenum sulfide, vanadium pentoxide, silicon,doped-graphene, graphene oxide, hydrogenated graphene, fluorinatedgraphene, a covalent organic framework, a layered transition metaldichalcogenide, a layered Group-IV and Group-III metal chalcogenide,silicene, germanene, and a layered binary compound of a Group IV elementand a Group III-V element.
 3. The method of claim 1, wherein theatomically thin active layer includes a plurality of defects in fluidcommunication with a portion of the second plurality of pores of theporous substrate, and further comprising depositing a second material inthe portion of the second plurality of pores in fluid communication withthe plurality of defects of the atomically thin active layer to at leastpartially fill the portion of the second plurality of pores.
 4. Themethod of claim 3, wherein the second material comprises at least one ofalumina, ruthenium oxide, zinc oxide, titania, zirconia, and hafnia. 5.The method of claim 3, wherein the second material deposited in theportion of the second plurality of pores increases a flow resistance ofthe portion of the second plurality of pores.
 6. The method of claim 1,further comprising modifying the porous substrate by plasma treating theporous substrate, irradiating the porous substrate with electrons and/orphotons, modifying the porous substrate with one or more functionalgroups, and/or grafting a polymer onto the porous substrate to decreaseleakage therethrough.
 7. The method of claim 6, wherein the modifiedporous substrate is bonded to the first material.
 8. The method of claim6, wherein a portion of the second plurality of pores of the modifiedporous substrate are uncovered by the atomically thin active layer, andfurther comprising reacting the modified porous substrate with one ormore materials configured to block leakage through the portion of thesecond plurality of pores of the modified porous substrate that areuncovered by the atomically thin active layer.
 9. The method of claim 1,wherein the first material is deposited using atomic layer deposition.10. The method of claim 1, wherein the membrane is permeable.
 11. Amembrane comprising: a porous substrate; a first layer comprising afirst material disposed on at least a portion of a surface of the poroussubstrate, wherein the first material is less hydrophilic than theporous substrate; and an atomically thin active layer disposed on thefirst layer, wherein the atomically thin active layer includes a firstplurality of open pores, and wherein the porous substrate includes asecond plurality of open pores that pass through the porous substrateand the first material such that the first plurality of pores and thesecond plurality of pores are in fluid communication such that a gasand/or liquid is capable of flowing through the membrane.
 12. Themembrane of claim 11, wherein the atomically thin active layer comprisesat least one of graphene, hexagonal boron nitride, molybdenum sulfide,vanadium pentoxide, silicon, doped-graphene, graphene oxide,hydrogenated graphene, fluorinated graphene, a covalent organicframework, a layered transition metal dichalcogenide, a layered Group-IVand Group-III metal chalcogenide, silicene, germanene, and a layeredbinary compound of a Group IV element and a Group III-V element.
 13. Themembrane of claim 11, further comprising a second material deposited ina plurality of pores of the porous substrate connected with a pluralityof defects of the atomically thin active layer to at least partiallyfill the plurality of pores of the porous substrate.
 14. The membrane ofclaim 13, wherein the second material comprises at least one of alumina,ruthenium oxide, zinc oxide, titania, zirconia, and hafnia.
 15. Themembrane of claim 13, wherein a thickness of the second materialdeposited in the plurality of pores of the porous substrate is selectedto provide a desired flow resistance of the porous substrate.
 16. Themembrane of claim 11, wherein the atomically thin active layer comprisesa plurality of atomically thin active layers.
 17. The membrane of claim16, further comprising pores in the plurality of atomically thin activelayers that are substantially aligned with one another.
 18. The membraneof claim 11, wherein the first material is a nanomaterial, wherein thefirst material is different than a material of the atomically thinactive layer.
 19. The membrane of claim 11, further comprising aprotective layer disposed on the atomically thin active layer.
 20. Themembrane of claim 19, wherein the protective layer is an anti-foulinglayer.
 21. The membrane of claim 11, wherein the first plurality of openpores are functionalized.
 22. The membrane of claim 11, wherein thefirst material is an atomic layer deposited material, wherein the firstmaterial is different than a material of the atomically thin activelayer.
 23. The membrane of claim 11, wherein the membrane is permeable.24. A membrane comprising: a porous substrate; a first layer comprisinga first material disposed on at least a portion of a surface of theporous substrate; and an atomically thin active layer disposed on thefirst layer, wherein a bond between the atomically thin active layer andthe substrate with the first layer is improved relative to a bondbetween the atomically thin active layer and the substrate without thefirst layer, wherein the atomically thin active layer includes a firstplurality of open pores, and wherein the porous substrate includes asecond plurality of open pores that pass through the porous substrateand the first material such that the first plurality of pores and thesecond plurality of pores are in fluid communication such that a gasand/or liquid is capable of flowing through the membrane, and whereinthe first material is different than a material of the atomically thinactive layer.
 25. The membrane of claim 24, wherein the first materialis less hydrophilic than the porous substrate.